I struggled long and hard with this sentence, but I'm almost there, I think, except for the last bit -καὶ τούτου γε δήπου τίς ἂν ἐπιθυμήσειεν;- What is τίς doing here? It must be an enclitic indefinite pronoun/ adjective (instead of an interrogative pronoun); but if adjective, I do not see what it is referring to, and if a pronoun I don't understand how it fits in the sentence.

I'm not sure I understand the whole passage, but isn't τίς in the final clause an interrogative pronoun? -- "who would desire that?" τίς with acute accent is interrogative, and the clause ends with a question mark.

As Qimmik noted, ἐπιθυμήσειεν is 3rd sg. opt. and it would be reasonable to assume τίς is its interrogative subject. The sentence ends with ; which is the Greek conventional question-mark.

From what I understand, the gist of the sentence is Socrates' statement that people who alreadypossess certain physical attributes, still wish to possess these attributes. But, he asks, who would desireto have what is already in his/her grasp?

According to R.G. Bury, the paragraph leading to 200c, is about wanting and desiring pertainingto the future -- keeping/maintaining what we now possess --, not to the present.

I struggle to explain δήπου here. This question does not, as stated in Smyth 2850, and is often the case, expects a positive answer. Its sole purpose is to oppose any such notionthat these people's desires pertain to the present. Harold N. Fowler (Cambridge, 1925) read itas "and how, I ask, is a man going to desire that?" He then immediately inserted "No" for the followingἀλλ᾽ ὅταν τις...

The gist of the argument appears to be something like this: it is (logically) necessary that an X man (fill in strong/ fast/ healthy) is X (strong/ fast/ healthy) whether he wants it or not, and desiring what is a (logical) necessity is absurd. But it is not a necessity that this situation will last in the future. So when an X man says he wants to be X, he actually means that he wants to remain X.